Low-cost wdm source with an incoherent light injected fabry-perot laser diode

ABSTRACT

The present invention discloses a low-cost light source for optical transmission systems and optical networks based on wavelength-division multiplexing (WDM) technology. A light source in accordance with the present invention is implemented by externally injecting a narrow-band incoherent light into a Fabry-Perot laser diode (F-P LD). After injection of narrow-band incoherent light, the output of F-P LD becomes wavelength-selective rather than multi-mode and the output wavelength of F-P LD coincide with the peak wavelength of the injected incoherent light. 
     Multi-channel WDM light sources according to the present invention can be implemented using a single broadband incoherent light source and plurality of F-P LDs. An optical transmission system for upstream signal transmission in an passive optical network using the light source according the present invention is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light source for optical transmissionsystems and optical networks based on the wavelength-divisionmultiplexing (WDM) technology, and more particularly to a light sourceemploying a Fabry-Perot laser diode (F-P LD) whose output wavelength islocked by an externally injected incoherent light.

The present invention also relates to WDM transmission systems and WDMpassive optical networks using the above-described light sources.

2. Description of the Related Art

Recently, WDM transmission systems are widely deployed to meet theever-growing bandwidth demands incurred by the explosion of the datatraffic. In particular, WDM transmission systems begin to be deployed inmetropolitan networks and access networks to accommodate wide-bandservices such as an electronic commerce, a cable TV, a video conference,and son on.

FIG. 1 shows a configuration of conventional WDM transmission system.

The source node is equipped with multiple transmitters (TXs) withdifferent wavelengths (λ₁˜λ_(N)) and a N×1 multiplexer (MUX) and thedestination node is equipped with an 1×N demultiplexer (DMUX) andmultiple receivers (RXs). The source node and the destination node areconnected through an single strand of optical link composed of opticalfibers and optical amplifiers.

In WDM transmission systems described above, communication channelsbetween the source node and the destination node are distinguished oneanother by their wavelengths. Thus, a unique wavelength is allocatedeach transmitter-receiver pair. The light, source of transmitter musthave the unique wavelength with long-term stability and a large sidemode suppression ratio (SMSR) to minimize the interference betweenneighboring channels. In addition, it is desirable that the light sourceprovides a sufficient output power and has a narrow spectral width.

A representative light source which satisfies the requirements mentionedabove is a distributed feedback laser diode (DFB LD). However, since adistributed feedback laser diode is expensive, incoherent light sourcesare usually used in an access network in which the main concern is theeconomical competitiveness

The incoherent light sources, such as a light emitting diode (LED), asuper-luminescent diode (SLD), and an optical fiber amplifier generatingamplified spontaneous emission (ASE), have been used in WDM transmissionsystems through a spectrum-slicing application. The LED can befabricated at low cost and modulated directly. However, the output powerof LED is not sufficient to accommodate many channels through aspectrum-slicing application. The SLD is costly although it can providemuch higher output power than the LED. The optical fiber amplifier canprovide a strong incoherent light, ASE, but it requires expensiveexternal modulators.

The F-P LD can provide much higher output power than the LED at thecomparable cost with the LED. However, its output is multi-mode and theoutput power of each mode fluctuates randomly with the time due to themode hopping and the mode partitioning. Therefore, it has been used inoptical transmission systems based on time-division multiplexingtechnology (TDM) rather than WDM technology. Its application wavelengthregion was also limited near the zero dispersion wavelength of theoptical fiber.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a low-cost lightsource for WDM application. The light source according to the presentinvention is implemented by externally injecting a narrow-bandincoherent light into a F-P LD. Its output is wavelength-locked by theexternally injected light and thus becomes wavelength-selective.

The other objective of the present invention is to provide WDMtransmission systems and WDM passive optical networks employing thelight source according to the present invention. The multiple slicedincoherent lights generated from a single broadband incoherent lightsource are injected into multiple F-P LDs simultaneously to producemulti-channel WDM light sources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a configuration of conventional WDM transmission system.

FIG. 2 shows the schematic diagram of the light source according to thepresent invention.

FIG. 3 is a schematic diagram of multi-channel WDM light sources inaccordance with the present invention.

FIG. 4 a and FIG. 4 b show schematically the optical transmissionsystems for upstream signal transmission in passive optical networksemploying the light source in accordance with an embodiment of thepresent invention.

FIG. 5 shows the experimental set-up to demonstrate the feasibility ofthe light source in accordance with the present invention.

FIG. 6 shows (a) the output spectrum of the F-P LD without externallight injection and (b) the spectrum of the narrow-band ASE to beinjected into the F-P LD.

FIG. 7 shows the measured output spectra of the F-P LD after injectionof a narrow-band ASE when the injection ASE power were (a) −2 dBm and(b) 2 dBm, respectively.

FIG. 8 shows the measured side-mode-suppression-ratio (SMSR) of thelight source in accordance with the present invention.

FIG. 9 shows the measured output spectra of the light source inaccordance with the present invention for different bias currents.

FIG. 10 shows the measured the extinction ratio of the light source inaccordance with the present invention.

FIG. 11 shows the measured output spectra of the light source inaccordance with the present invention when a polarizer and apolarization controller were further used.

FIG. 12 shows the measured bit error rate.

DESCRIPTION OF THE NUMERICS ON THE MAIN PARTS OF THE DRAWINGS

TX: a transmitter

RX: a receiver

MUX: a multiplexer

DMUX: a demultiplexer

ILS: an incoherent light source

TF: a tunable optical filter

CIR: an optical circulator

Pol: a polarizer

PC: a polarization controller

F-P LD: a Fabry-Perot laser diode

ILS: an incoherent light source

BPF: a band pass filter

(D)MUX: (de)MUX

ASE source: an ASE source

WGR: a waveguide grating router

AMP1, AMP2: an optical amplifier

Att.1, Att.2: an optical variable attenuator

PZF: a polarizing fiber

SMF: a conventional single mode fiber

PM: an power meter

DETAILED DESCRIPTION OF THE EMBODIMENTS

It is well known that the F-P LD shows a multi-mode output and the modepower is proportional to the spontaneous emission coupled to the mode.The output spectral distribution of the F-P LD can be changed byexternally injecting a strong light into the F-P LD. Then, a mode thatis the nearest from the peak wavelength of the injected light is lockedby the injected light and the other modes may be suppressed. Namely, theoutput wavelength of F-P LD coincides with the peak wavelength injectedlight. As a result we can obtain a wavelength-selective output frommulti-mode laser, F-P LD.

Hereinafter, referring to appended drawings, desirable embodiments ofthe present invention are described in detail.

FIG. 2 is a schematic diagram of the light source according to theembodiment of the present invention. The light source comprises: anincoherent light source (ILS); a tunable optical filter (TF) connectedto said incoherent light source; an optical circulator (CIR) connectedto said tunable optical filter; and a F-P LD without optical isolatorconnected to said optical circulator.

Optionally, the light source according to the embodiment of the presentinvention further comprises: a polarization controllers (PC) connectedbetween said optical circulator and said F-P LD; and a polarizer (Pol)connected at the output end of said optical circulator.

In the embodiment, the incoherent light source is any one of an opticalfiber amplifier generating ASE, an LED, or a SLD.

The operation principles of the light source according to the presentembodiment are as follows:

The broadband incoherent light generated from the incoherent lightsource is sliced by the tunable optical filter to produce a narrow-bandincoherent light. The narrow-band incoherent light is injected into theF-P LD through the optical circulator. The optical circulator separatesthe narrow-band incoherent light and the output of F-P LD. Thus theoutput of the light source according to the present embodiment comes outthrough the output end of the optical circulator.

When the F-P LD is biased above the threshold current, the output of theF-P LD is multi-mode. However, it becomes wavelength-selective afterinjection of the narrow-band incoherent light since a strong light iscoupled to a specific mode of the F-P LD. The output wavelength of F-PLD is locked to the injected incoherent light and thus can be tuned bychanging the pass-band of the tunable optical filter.

The output power of the F-P LD can be changed by controlling the biascurrent applied to the F-P LD. Thus, we can modulate the light sourcedirectly. When the bias current is lower than the threshold current, theoutput of the light source is a reflected incoherent light at theinterface of the pig-tailing fiber and the air. The output of F-P LD ispolarized but reflected incoherent light is unpolarized. Using thischaracteristics, the extinction ratio of the modulated signal can beimproved by further comprising a polarization controller (PC) and apolarizer (Pol).

In the light source according to the present embodiment, an opticalcirculator (CIR) can be replaced by an optical power splitter.

Using the same principles as that of the embodiments described above,multi-channel WDM light source can be implemented.

FIG. 3 shows schematic diagram of the multi-channel WDM light source inaccordance with the embodiment of the present invention.

The multi-channel WDM light source comprises: an incoherent light source(ILS); an optical circulator (CIR) connected to said incoherent lightsource; a (de)multiplexer ((D)MUX) connected to said optical circulator;and plurality of F-P LDs without optical isolator connected at theoutput end of the said (de)multiplexer.

If the bandwidth of the incoherent light generated said incoherent lightsource is larger than the free spectral range (FSR) of said(de)multiplexer, the light source further comprises a band-pass filter(BPF) that is connected between said optical circulator (CIR) and said(de)multiplexer. The band-pass filter restricts the bandwidth of theincoherent light entering the (de)multiplexer within the free spectralrange (FSR) of an the (de)multiplexer.

Optionally, the light source further comprises: plurality ofpolarization controllers (PC) connected between the output ends of thesaid (de)multiplexer and said F-P LDs; and a polarizer (Pol) connectedat the output end of said optical circulator. In the embodiment, theincoherent light source is any one of an optical fiber amplifiergenerating ASE, an LED, or a SLD.

The operation principles of the multi-channel WDM light source in thepresent embodiment is as follows: The broadband incoherent lightgenerated from the incoherent light source is transmitted to the(de)multiplexer through the optical circulator. The (de)multiplexerreceives and slices the broadband incoherent light. Then, the slicednarrow-band incoherent light with different wavelengths are injectedsimultaneously into the plurality of F-P LDs.

After injection of incoherent light, the output of each F-P LD becomeswavelength-selective and is locked by the injected narrow-bandincoherent light. Namely, the output wavelength of each F-P LD coincideswith the peak wavelength of the (de)multiplexer pass-band. The outputsof the F-P LDs are multiplexed by the (de)multiplexer. Then, themulti-channel WDM signals come out through the output end of the opticalcirculator.

The output power of multi-channel WDM light source can be controlledindependently and thus multi-channel WDM light source can be modulateddirectly. We can increase the extinction ratio of the modulated signalby further comprising a polarizer (Pol) and plurality of polarizationcontrollers (PC).

In the multi-channel WDM light source according to the presentembodiment, an optical circulator (CIR) can be replaced by an opticalpower splitter.

FIG. 4 a shows a schematic diagram the optical transmission system forupstream signal transmission in a passive optical network using themulti-channel WDM light source in accordance with the present invention.

The passive optical network of the present embodiment comprises acentral office, a remote node connected to the central office with asingle optical fiber, and plurality of optical network units connectedto the remote node with plurality of optical fibers;

wherein the central office comprises: an incoherent light source (ILS);a demultiplexer (DMUX); an optical circulator that route the output ofsaid incoherent light source to the optical fiber connecting saidcentral office and said remote and the upstream signal transmitted fromsaid remote through said optical fiber to said demultiplexer; andplurality of receivers (RX) connected at the output ends of the saiddemultiplexer,

the remote node comprises: an (de)multiplexer that receives thebroadband incoherent light transmitted from said central offices, slicessaid incoherent light spectrally to produce plurality of narrow-bandincoherent lights and multiplexes the upstream signals from said opticalnetwork units, and

the plurality of optical network units comprise a F-P LD that isconnected to the output ends of the (de)multiplexer in the remote nodewith said plurality of optical fibers.

Under this configuration, the upstream signals generated from theoptical network units have different wavelengths and multi-channel WDMsignal is transmitted from the remote node to the central office.

In the passive optical network, electric power is not supplied to theremote node to save the maintenance cost, and thereby the pass-band ofthe (de)multiplexer in remote node can drift with the temperaturechange. Therefore, it is important to control the wavelength of thelight sources in the optical network units. In case of the passiveoptical network using the multi-channel WDM light source according tothe present invention, the output wavelength of each F-P LD isautomatically aligned to the pass-band of the (de)multiplexer in remotenode since the output wavelength of the F-P LD is locked by the injectedincoherent light.

In the passive optical network described above, the broadband incoherentlight transmitted from the central office to the remote node may bereflected to the central office due to the Rayleigh back-scattering ofthe optical fiber. The reflected light can degrade the signal quality.

FIG. 4 b shows a schematic diagram of the optical transmission systemfor upstream signal transmission in a passive optical network to reducethe signal degradation described above.

As described in the figure, by installing an optical circulator (CIR) atthe remote node and separating the optical fiber that delivers theincoherent light from the optical fiber that deliver the upstreamsignal, the signal degradation caused by the reflection of theincoherent light can be reduced.

In other words, the passive optical network of the present embodimentcomprises a central office, a remote node connected said central officewith two optical fibers, and plurality of optical network unitsconnected to said remote node with plurality of optical fibers;

wherein the central office comprises: an incoherent light source (ILS)connected to said remote node with an optical fiber; a demultiplexer(DMUX) connected to said remote with the other optical fiber andplurality of receivers (RX) connected at the output ends of the saiddemultiplexer,

the remote node comprises: a (de)multiplexer that receives the broadbandincoherent light transmitted from the central offices, slices saidincoherent light spectrally to produce plurality of narrow-bandincoherent lights, and multiplexes the upstream signals from saidoptical network units; and an optical circulator that route thebroad-band incoherent light transmitted from said central office to said(de)multiplexer and the upstream signals from said (de)multiplexer tothe central office, and

the plurality of optical network units comprise F-P LDs connected to theoutput ends of the (de)multiplexer in the remote node with saidplurality optical fibers.

Under this configuration, the upstream signals generated from theoptical network units have different wavelengths and multi-channel WDMsignal is transmitted from the remote node to the central office.

In optical transmission system for upstream signal transmission in apassive optical network described in FIG. 4 a and FIG. 4 b, an opticalcirculator (CIR) can be replaced by an optical power splitter.

FIG. 5 shows the experimental set-up to demonstrate the feasibility ofthe light source in accordance with the present invention.

The ASE source was two-stage erbium-doped fiber amplifier (EDFA) pumpedcounter-directionally with laser diode at 1480 nm. The pump power forthe first and the second stage of EDFA were 50 mW and 100 mW,respectively. A band pass filter (BPF) with a bandwidth of 9 nm was usedat the output end of the EDFA to limit the spectral width of the ASEwithin one free spectral range (FSR) of the waveguide grating router(WGR). An optical amplifier (AMP1) and an optical variable attenuator(Att.1) were used to control the ASE power injected into the F-P LD. Anoptical circulator with insertion loss of 0.7 dB separated the injectedbroadband ASE and the output of the F-P LD. The broadband ASE was slicedspectrally by an WGR with a bandwidth of 0.24 nm and injected into theF-P LD. A conventional F-P LD without an optical isolator was locked bythe externally injected narrow-band ASE. The threshold current of theF-P LD was 20 mA. The coupling efficiency of the F-P LD, the rate ofpower transferred from laser to pig-tailing fiber or vice versa, wasapproximately 8%. The F-P LD was modulated directly by pseudorandomnonreturn-to-zero data with a length of 2⁷−1 at 155 Mb/s and its outputwas transmitted through conventional single mode fiber (SMF). Thetransmitted data was amplified by an optical amplifier (AMP2),demultiplexed by another WGR with a bandwidth of 0.32 nm, and receivedby a PIN photo-detector based receiver to measure the bit error rate(BER) characteristics. The receiver input power was controlled by anoptical variable attenuator (Att.2) and measured by an optical powermeter (PM). A polarization controller (PC) and a polarizing fiber (PZF)with about 47 dB of polarization extinction ratio are used to improvethe extinction ratio of the modulated optical signal.

FIG. 6 shows (a) the output spectrum of the F-P LD without ASE injectionand (b) the spectrum of the narrow-band ASE to be injected into the F-PLD. The bias current was 30 mA and the output power of the F-P LDmeasured at the output end of the optical circulator was about −10 dBm.The side mode suppression ratio (SMSR) was less than 6 dB. The peakwavelength of narrow-band ASE was about 1551.72 nm.

FIG. 7 shows the measured output spectra of the F-P LD after injectionof a narrow-band ASE when the injected ASE power were (a) −2 dBm and (b)2 dBm, respectively. After ASE injection, the F-P LD waswavelength-locked by the injected ASE. The measured side modesuppression ratio were 25 dB and 27.3 dB for the injection ASE power of−2 dBm and 2 dBm, respectively.

FIG. 8 shows the measured side mode suppression ratio (SMSR) of thelight source in accordance with the present invention. The side modesuppression ratios increases as the injected ASE power increases.However, it decreases as the bias current increases.

To measure the modulation characteristics of the light source inaccordance with the present invention, we measured optical spectra fordifferent bias currents at the fixed injection ASE power of 2 dBm. FIG.9 shows the results when the bias current were 30 mA (dotted line) and 0mA (solid line), respectively. The measured peak power differencebetween two bias states, here called as extinction ratio, was about 5.8dB.

FIG. 10 shows the measured the extinction ratio of the light source inaccordance with the present invention. The extinction ratio decreases asthe injection ASE power increases while it increases the as the biascurrent increases.

We also measured optical spectra by inserting a polarization controllerand a polarizer (in the present experiment, a polarizing fiber: PZF)under the same measurement conditions with the FIG. 9. FIG. 11 shows theresults. The extinction ratio increases about 2.5 dB from 5.8 dB to 8.3dB. This means that the output of the light source according to thepresent invention is polarized.

FIG. 12 shows the measured bit error rate curves. The F-P LD wasmodulated directly at 155 Mb/s. The amplitudes of dc bias and modulationcurrent were both 20 mA. Before we use the light source according to thepresent invention, we measured BER characteristics of the directlymodulated F-P LD itself, i.e., without ASE injection. The measured powerpenalty at the BER of 10⁻⁹ was about 2 dB after transmission over 20 kmof SMF as shown in FIG. 12 (a).

The BER characteristics were improved dramatically when we inject anarrow-band ASE into the F-P LD. The power and the peak wavelength ofthe injected ASE were 1 dBm and 1551.72 nm, respectively. We achievederror free transmission over 120 km of SMF with negligible power penaltyas shown in FIG. 12 (b). We also measured BER characteristics bychanging the peak wavelength of the injected narrow-band ASE andobserved very similar results. As an example, we show the measured BERcurves in FIG. 12 (c) when the peak wavelength of the injectednarrow-band ASE was 1550.92 nm. This result implies that the outputwavelength of the light according to the present invention can be tunedby changing the wavelength of the injected ASE.

Since those having ordinary knowledge and skill in the art of thepresent invention will recognize additional modifications andapplications within the scope thereof, the present invention is notlimited to the embodiments and drawings described above.

1-14. (canceled)
 15. A multi-channel WDM light source comprising: anincoherent light source that generates a broadband incoherent light; anoptical circulator coupled to the incoherent light source; ademultiplexer coupled to the optical circulator, wherein the opticalcirculator is configured to route the broadband incoherent light to thedemultiplexer and to separate an output of the demultiplexer from thebroadband incoherent light, wherein the demultiplexer is configured toreceive the broadband incoherent light, to slice spectrally thebroadband incoherent light to produce a plurality of narrow-bandincoherent lights; and a plurality of light sources capable of lasingthat are connected at output ends of the demultiplexer, wherein thedemultiplexer is further configured to multiplex outputs of theplurality of light sources capable of lasing, wherein at least one ofthe plurality of light sources capable of lasing is configured to emit awavelength-selective output locked by one of the narrow-band incoherentlights, wherein the at least one of the plurality of light sourcescapable of lasing is configured to be modulated directly.
 16. Themulti-channel WDM light source of claim 15, wherein the plurality oflight sources comprise Fabry-Perot laser diodes.
 17. The multi-channelWDM light source of claim 15, wherein the broadband incoherent light hasa bandwidth within a free spectral range (FSR) of the demultiplexer. 18.The multi-channel WDM light source of claim 15, further comprising aplurality of polarization controllers between the output ends of thedemultiplexer and the plurality of light sources.
 19. The multi-channelWDM light source of claim 15, further comprising a polarizer coupled toan output of the optical circulator.
 20. The multi-channel WDM lightsource of claim 15, wherein the optical circulator is replaced by anoptical power splitter.
 21. The multi-channel WDM light source of claim15, further comprising a band pass filter between the demultiplexer andthe optical circulator.
 22. The multi-channel WDM light source of claim15, wherein a generator is used to directly apply a current to the atleast one of the light sources capable of lasing to modulate data ontothe wavelength-selected outputs.